1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and more particularly to a method of preparing an electrophotographic photosensitive member by use of a coating solution having good stability, and a method of preparing it.
2. Related Background Art
Electrophotography employs photoconductive materials comprising a support coated with a material having insulating properties in the dark and capable of changing its electrical resistance responding to the amount of irradiation received in the course of image exposure, as disclosed in U.S. Pat. No. 2,297,691. Basic characteristics required in electrophotographic photosensitive members using this photoconductive material are that (1) the photosensitive members can be charged to suitable potential in the dark, (2) they suffer less diffusing-away of electric charges in the dark, (3) the charge can be quickly diffused away by irradiation of light.
As electrophotographic photosensitive members, there have hitherto widely used inorganic photosensitive members having a photosensitive layer mainly comprised of an inorganic photoconductive compound such as selenium, zinc oxide and cadmium sulfide. These, however, though satisfying the above conditions (1) to (3), can not necessarily be satisfactory in the thermal stability, humidity resistance, durability, productivity, etc. For example, selenium, once it is crystallized, brings about deterioration of the properties as a photosensitive member, causing difficulties in the manufacture, and may be crystallized because of heat or fingerprints to deteriorate the performances as a photosensitive member. Also, in the case of cadmium sulfide, there are problems in the humidity resistance and durability, and, in the case of zinc oxide, in the smoothness, hardness and wear resistance. Moreover, many of inorganic photosensitive members have limits in photosensitivity wavelength regions. For example, selenium has a photosensitivity wavelength region in the blue region, and has little sensitivity to the red region. For this reason, various methods are proposed to expand the photosensitivity to a longer wavelength region, but with many limitations in selecting a photosensitivity wavelength region. Also when zinc oxide or cadmium sulfide is used in photosensitive members, it has in itself a narrow photosensitivity wavelength region, thus requiring addition of various sensitizing agents.
For the purpose of overcoming the disadvantages involved in these inorganic photosensitive members, development is recent years extensively made on electrophotographic photosensitive members mainly comprised of a variety of organic photoconductive compounds. For example, they include a photosensitive member having a charge generation layer and a charge transport layer containing triallylpyrazoline as disclosed in U.S. Pat. No. 3,871,882, and a photosensitive member having a charge generation layer comprising a derivative of perylene pigments and a charge transport layer comprising a condensate of 3-bromopyrene with formaldehyde as disclosed in U.S. Pat. No. 3,837,851. Also already known as photosensitive members employing bisazo pigments or trisazo pigments as charge-generating materials are those disclosed in Japanese Patent Laid Open Applications Nos. 59-33445, 56-46237, 62-111294, etc. The organic photoconductive compounds can also freely change the spectral sensitivity of the photosensitive members employing the compounds. For example, a great number of publications teach that the photosensitive members employing phthalocyanine compounds have sensitivity nearly up to 800 nm. Also, azo pigments disclosed in Japanese Patent Laid-Open Applications Nos. 61-272754 and 56-167759 have a high sensitivity in the visible light region, and Japanese Patent Laid-Open Applications Nos. 57-195767 and 61-228453 disclose those having sensitivity up to the infrared region.
The organic photoconductive compounds also have advantages not only in the superiority in electrophotographic characteristics but also particularly in productivity. For example, the photosensitive members employing the inorganic compounds such as selenium and the amorphous silicon that is recent years attracting notices as a material for electrophotographic photosensitive members can be prepared at present time only by production methods having a low production efficiency as exemplified by vapor deposition and sputtering. In contrast with this, however, the electrophotographic photosensitive members employing the organic photoconductive compounds in the charge generation layer can be prepared by using simple coating methods.
As stated above, the electrophotographic photosensitive members employing the organic photoconductive compounds in the charge generation layer have many advantages in the electrophotographic characteristics and productivity. Particularly in regard to the productivity, they are far beyond the electrophotographic photosensitive members employing the inorganic compounds.
The reason why they have high productivity is that the charge generation layer can be formed on a support by use of simple coating methods as mentioned above, but required for that purpose is a coating solution in which the charge-generating materials have been homogeneously dispersed in a solvent. Particularly to carry out coating methods suited for mass production, as exemplified by dip coating, blade coating and Meyer bar coating, the charge-generating materials dispersed in the coating solution are required to be stable. Coating solutions actually used are prepared by dispersing the organic photoconductive compounds as described above and binders (such as polymer resins) with use of a dispersing means such as a ball mill, a sand mill and an attriter. This dispersing method may differ depending on various organic photoconductive compounds (i.e., organic pigments), but the fact is that optimum conditions (such as dispersing time and grinding power in dispersing) are often unknown before the dispersing is actually carried out, and appropriate dispersing conditions can be found with difficult unless trial and error are repeated, requiring much labor. The phenomenon that particle size is easily enlarged if the materials are dispersed to have a given particle size or that the viscosity of solutions becomes extraordinarily high, i.e., the stability of coating solutions, may also differ. This also depends on trial and error to be repeated and may differ depending on the organic photoconductive compounds to be used. Accordingly, it has been difficult to seek out the dispersing conditions under which an optimum stability can be shown. In the end, what has been hitherto aimed in the dispersing step is to atomize the organic pigments, and it is commonly practiced to stop the dispersing when a given particle size was achieved.
It has often occurred that allowing the coating solutions thus prepared to stand for storage or carrying out circulation or the like to actually make electrophotographic photosensitive members results in enlarging grain size with time or increase in viscosity to make the coating solutions unusable.